Many techniques have been attempted for monitoring or controlling resistance spot welds during the welding operation. These have included sensing weld resistance, acoustic emissions, weld tip displacement, and ultrasonic testing. Very few of the attempts have been commercially successful and any successes have been limited to narrow limits of operation or nearly ideal welding operations. Environments such as automotive manufacture provide less than ideal condition and varied parameters so that different metal thicknesses, number of layers, and metal coatings all challenge the ability of a given approach to weld control.
Ultrasonics have been tried for weld monitoring with no real success prior to this invention. The usual ultrasonic method is to try to track the weld nugget growth by transmitting ultrasonic energy through the weld and analyzing the signal amplitude variations. The patent to Burbank et al U.S. Pat. No. 3,384,733 is an example of this type.
Weld tip displacement has been used to track the progress of a weld and terminate the weld current when the displacement indicates that indentation has occurred. This is shown by Steibel et al, "Monitoring and control of spot Weld Operations", Society of Automotive Engineers, 1986. In that case the only control is the termination time, the current being controlled by a preset schedule.